WO 02/04119A1 discloses a process for trimerization of olefins comprising contacting a monomeric olefin or a mixture of olefins under trimerization conditions with a catalyst, which comprises (a) a source of chromium, molybdenum or tungsten, (b) a ligand containing at least one phosphorus, arsenic or antimony atom bound to at least one hydrocarbyl or heterohydrocarbyl group having a polar substituent, but excluding the case where all such polar substituents are phosphane, arsane or stibane groups, and optionally (c) an activator.
Also, WO 2004/056477A1 discloses a process for trimerisation of olefins, which process includes the steps of contacting an olefinic feedstream with a catalyst system which includes a transition metal compound and a heteroatomic ligand and wherein the trimer is an olefin and wherein the heteroatomic ligand is described by the following general formula (R)nA-B-C(R)m.